1. Field of the invention
The present invention relates to an organic positive temperature coefficient (PTC) thermistor. More specifically, the present invention relates to an organic PTC thermistor in which an electrode made from conductive paste is formed on a main surface of an organic PTC thermistor sheet.
2. Description of the Background art
An organic resin which the nature of a PTC thermistor is obtained by mixing and distributing conductive particles such as carbon black, graphite, metallic powder or the like in a polyolefin-family resin, such as polyethylene. It is known that an organic PTC thermistor is obtained by forming such an organic PTC thermistor resin as an organic PTC thermistor sheet and by forming a pair of electrodes on a main surface thereof.
In order to form a pair of electrodes in such an organic PTC thermistor, there exist a method wherein metallic foil is adhered on the main surface of the organic PTC thermistor sheet and the electrodes having a predetermined pattern are formed by etching the metallic foil, and a method wherein electrodes having a predetermined pattern are formed on the main surface of the organic PTC thermistor sheet by screenprinting conductive paste thereon.
In an etching method, there the possibility that the organic PTC thermistor sheet is deteriorated by an etching solution. In view of this point, a method where the electrodes are formed by the conductive paste is superior to the etching method.
However, in the method where the; electrodes are formed by conductive paste the following problem occurs the following takes place. Specifically, the organic PTC thermistor sheet is normally obtained by extrusion molding or press molding of the organic PTC thermistor resin. In molding, lack of uniformity of distribution of the conductive particles takes place in the organic PTC thermistor sheet such that the density of conductive particles at the surface of the organic PTC thermistor sheet (i.e. surface density) is lowered. Therefore, in the case where the electrodes are formed by painting or printing the conductive paste on the surface of such an organic PTC thermistor sheet as it is, a resistance value between the electrodes of the organic PTC thermistor becomes very large in comparison with the inherent resistance constant of the organic PTC thermistor sheet, and there results a large dispersion of the resistance value due to dispersion of the surface density of the conductive particles at the surface of the organic PTC thermistor sheet. Therefore, in the case where the organic PTC thermistor is used as a heating plate, heating temperature cannot become even in the whole surface of the organic PTC thermistor sheet.